1. Field of the Invention
The field of the invention relates to stereophonic frequency modulation (FM) companding or compression systems.
2. Prior Art
The FMX (TM) companding system is a noise reduction system for FM stereo radio which was developed jointly by Thomas Keller of the National Association of Broadcasters and Emil Torick of CBS Technology Center.
The conventional FM stereo system employs a 38 kHz double-sideband suppressed-carrier AM subchannel to carry the stereo difference signal (L-R). A 19 kHz pilot tone provides a phase reference to regenerate the 38 kHz at the receiver, and also serves to indicate to the receiver that a stereo broadcast is in progress. The subchannel occupies the frequency region from 23 to 53 kHz on the baseband, while the sum (L+R) information occupies the 30-15,000 Hz region to assure compatibility with mono radios. Due to its limited modulation index, the L-R subchannel is far noisier than the main channel. When a receiver is switched from mono to stereo, the received signal becomes 23 to 26 dB noisier.
The FMX system attempts to remove most of this noise by adding a second subchannel in phase quadrature to the existing subchannel. The new "Q" subchannel contains a compressed version of the L-R information, which is also carried in the conventional in-phase "I" subchannel to assure compatibility. The compressed Q channel generally carries a higher level of modulation than does the conventional uncompressed I channel. This fact is exploited to produce noise reduction at the receiver.
The special FMX receiver decodes both the I and Q subchannels. It sums the decoded channels, and applies them to a voltage-controlled amplifier. A servo compares the level at the output of the variable gain amplifier with the level at the output of the I channel alone (which contains the L-R at the correct level and can thus be used as a reference). The servo forces the output of the voltage-controlled amplifier to be at the same level as the I channel alone. Because the Q channel ordinarily contains a higher level of modulation than the I channel, this usually results in the voltage-controlled amplifier's having a gain of less than unity. This reduces any noise picked up by the subchannels. This use of the I channel as a reference for the expander is referred to as "adaptive expansion", and permits the expander to accurately track the Q-channel compressor even if its characteristics (such as compression ratio and attack and release time constants) are varied over a wide range. This is in contrast to a conventional companding system, where accurate reconstruction of the signal requires accurately complementary compressor and expander characteristics.
One compression system has been proposed by CBS/NAB. (See "Improving the Signal-To-Noise Ratio and Coverage of FM Stereophonic Broadcasts" by Emil L. Torick, CBS Technology Center, Stamford, Connecticut, and Thomas B Keller, National Association of Broadcasters, Washington D.C.; AES Preprint No. 2119, October, 1984; and "Re-Entrant Compression and Adaptive Expansion For Optimized Noise Reduction", by Gravereaux, Stebbings, Kadin and Cugnini; CBS Technology Center, Stamford, Connecticut, Apr. 10, 1985.) In this system the L-R signal is compressed with an .infin.:1 slope, with the compression threshold set at -30 dB (100% modulation=+75 kHz carrier deviation). The input of a voltage-controlled amplifier employed in the compressor is summed out-of-phase with the amplifier's output to achieve a "re-entrant" characteristic. That is, as the L-R modulation approaches 100%, the level in the Q channel approaches 0 to avoid overmodulation of the overall composite baseband signal. Overmodulation avoidance in this system is essentially statistical: the level in the Q channel is reduced whenever there is a high probability that the baseband would otherwise be overmodulated. However, there are still instances which will cause overmodulation. A good example is a high-level source panned slightly left or right to center, such that almost full baseband modulation is produced (assuming no Q channel present), yet the L-R is small (because the L and R signals are almost equal in level). In this case, the CBX compressor can produce modulation of up to 126% when the Q channel is turned on, which violates FCC Rules. Clearly, it is necessary to take steps to control baseband modulation under all soundfield conditions.
While overall modulation of the main channel and I channel could be reduced approximately 2 dB to accommodate such potential overmodulation, this would cause loudness loss (and also cause the signal-to-noise ratio to deteriorate). A limiter could be placed at the composite output of the stereo generator to control modulation, but this would also cause loudness loss and might cause "pumping" or other obvious limiter-induced artifacts. A clipper could be placed at the composite output of the stereo generator, but this would cause increased distortion when it operated, and would also disturb the spectral integrity of the baseband by producing harmonic and intermodulation distortion in the frequency range byond 57 kHz. None of the these solutions to the problem of Q-channel-induced overmodulation is really satisfactory.
The present invention not only completely solves the overmodulation problem, but does so without affecting compatible reception (i.e. conventional receivers without FMX decoding), and while preserving the spectral integrity of the baseband. In essence, it uses a delay-line controller similar in concept to that described in U.S. Pat. No. 4,208,548. That is, a "pilot system" using feedback control is used to derive a control voltage for a voltage-controlled amplifier, the control voltage is delayed and smoothed, and is used to control a second voltage-controlled amplifier whose input is a delayed version of the input of the first voltage-controlled amplifier.